1. Field of the invention
The invention relates to an orthodontic device, with a ceramic tooth attachment of a polycrystalline ceramic structure and various additives with an optical transmittance in the visible spectrum at a thickness of 0.5 mm, in the range of from 5 to 60%.
2. Description of the prior art
In a known orthodontic device--according to U.S. Pat. No. 1,976,141 and U.S. Pat. No. 2,045,025, the tooth attachments are made of stainless steel and secured to a stainless steel band, which encircles the perimeter of the tooth in order to keep the tooth attachment in the right position.
Various other tooth alignment methods and orthodontic devices have been developed in which stainless steel tooth attachments are cemented directly to the surface of the tooth. These methods and devices require no band to secure the tooth attachments. Such systems are used very often. These known tooth alignment methods and devices are known among others from the U.S. Pat. Nos. 4,604,057, 4,430,061 and 4,322,206. The metallic appearance of such stainless steel tooth attachments is a significant drawback. Therefore, for reasons of appearance, especially adult patients desire orthodontic devices, which are not made of metal but of transparent and translucent materials as such materials provide improved cosmetic appearance. Accordingly, transparent plastic materials are used, mostly polycarbonates or transparent or translucent ceramic materials which comprise aluminum oxide. With known tooth attachments made of nonmetallic materials, which are produced in very small sizes in order to keep cosmetic drawbacks at a minimum, it is known--according to U.S. Pat. No. 4,302,532--that the strength of ceramic tooth attachments made of plastic materials is increased by reinforcing elements.
Furthermore, orthodontic devices with a ceramic tooth attachment consisting of a polycrystalline, ceramic structure with various additives are known. A known ceramic tooth attachment--according to U.S. Pat. No. 4,954,080--consists of polycrystalline ceramics with a light transmission in the visible spectrum, which reduces the visibility of this tooth attachment, so that when it is mounted on the tooth, it can be hardly recognized by a third party. This polycrystalline ceramic body for the ceramic tooth attachment is produced by pressing a high purity aluminum oxide powder, which, in the process, is sintered in order to present a close to zero porosity and an average grain size in the range of 10 to 30 micrometers. Preferably, the tooth attachment is colorless. An in-line light transmission in the visible spectrum should be 20 to 60% through a specimen having a thickness of 0.5 mm. The disadvantage with this tooth attachment is that the latter can not be seen when X-rays are made.
However, it is already known--according to EP-PS 0 189 540--that microfilled dental material is being manufactured which presents both, good X-ray opaqueness and at the same time good transparency. This dental material which is used as filling material, dental cement, crown and bridge material, prosthetic material and also for the production of dentures, inlays, implantations and prefabricated parts, consists of polymerized inorganic binders and compositions of rare precious metals and also, if required, further inorganic compositions as filling materials, whereby it may also contain a fluorinated rare-earth metal (elements of atomic number 57-71 of the periodic table of elements) or a mixture of these fluorides. This type of dental material is often mixed to a paste by kneading, which hardens under various conditions, in particular under the effect of light. The light transmission in the visible spectrum for such parts is mostly sufficient, however, when used for tooth attachments of orthodontic devices it was not satisfactory.